1. Field of the Invention
This invention concerns itself generally with the transmission of low frequency sub-audible tones over digital loop carrier, and related thereto, with providing alarm services over digital loop carrier (DLC). More specifically, it is concerned with the transmission of alarm related signals over DLC channels which simultaneously provide facilities for the subscriber's normal residence or business plain ordinary telephone service (POTS).
2. Description of the Prior Art
The specific type of alarm reporting and collecting system concerned transmits a pilot tone, at a frequency in the sub-audible range below the normal voice band, as an "all is well" indication to scanning equipment in the central office. The scanning equipment communicates with the subscriber terminal unit (STU) provided by an alarm company at the subscriber's premises by means of bi-directional frequency shift keying (FSK) data transmission, using carrier frequencies at the upper end of the telephone voice band, in the 2700 to 3000 Hz range. When the subscriber is on-hook, his STU is polled at regular intervals on the order of 30 seconds, regardless of the presence of the low frequency pilot tone, hereinafter referred to as "low tone", and either gives a positive "all is well" response if its alarm conditions are normal, or reports any abnormal conditions to the scanner.
Normally, the subscriber does not wish to be bothered by these audible tones when engaged in conversation or non-alarm related data transmission, or at least wants the frequency of their occurrence to be substantially reduced. Accordingly, when the subscriber is off-hook and low tone is present, the scanner either does not attempt to communicate with the STU at all, or only polls it at much longer intervals. If an alarm is registered by the STU, the STU cuts its low tone, and the scanner, on detection of low tone loss, scans the off-hook line. The scanner, or equipment associated with it, passes the alarm indication to the alarm company, which then initiates further communication with the STU, via the scanner, using the previously mentioned FSK data signals, with the central office located equipment relaying the information.
Since this alarm service uses the same transmission facilities as the subscriber's POTS service, it can be provided more cheaply than more conventional alarm systems such as McCulloh loops, which require a separate, dedicated facility. Prior to the present invention, however, this service could not be provided over DLC channels because they have excessive transmission losses at the low tone frequency, and often do not provide bidirectional voice frequency transmission in the on-hook state. Since the use of DLC by telephone companies to provide POTS is increasing rapidly, the incompatibility of conventional DLC channels with this type of alarm service is becoming increasingly troublesome to them. If a subscriber is currently served via DLC when the alarm service is ordered, they must either deny service, or switch the subscriber to a more expensive metallic facility, if in fact one is available. Worse yet, if the subscriber already has alarm service, he cannot be switched over to DLC without discontinuing his alarm service, which is extremely distasteful, and as a practical matter, probably impossible for the telephone company. Thus, it can be seen that inability to furnish alarm service as described herein via DLC causes maintenance, administrative, and public relations problems for telephone companies who include such service in their tariffed offerings.
For a DLC channel to be compatible with such alarm service, two characteristics are essential:
1. A one-way transmission path at the low tone frequency, with acceptable transmission loss, must be provided in both the on-hook and off-hook states, from the STU to the CO through the DLC channel,
2. A two-way voice band path for transmission of the FSK signals must be maintained, both in the on-hook and the off-hook states, through the DLC channel. It is a general requirement that DLC systems provide a two-way voice frequency transmission path at all times on non-concentrated channels.
It is desirable that any modification to the DLC so that it is compatible with such alarm service be in the form of circuitry which can be added as needed to both the subscriber and central office portions of the DLC. It is also desirable that any such added circuitry not impair the performance of the two-way path in the DLC. It is further desirable that any such circuitry not respond to the low tone in any way other than to ensure that it can be transmitted "as is" from the remote terminal to the central office. The circuitry will then not cause a spurious signals to be received which indicates the presence of the low tone when the same is not present. The system of the present invention not only allows the DLC to be compatible with such alarm service and meets the other requirements described above but is also in the form of circuitry that can be added to the DLC when needed.